Crystallization



Se t. 18, 1951 M 2',5s7,9ss'

CRYSTALLIZATION Filed may 1, 1948 2 Sheets-Sheet 1 I MATURE CRYSTALS WALTER (LSAEMAN INVENTOR.

Patented Sept. 18, 1951 UNITED PATENT OFFICE 2,567,968 ORYSFIALLIZATION ApplicationMay, 1, 1948, Serial No. 24,635

6 Claims. (01. 23302) (Granted under the act of March 3, 1883, as amended Aprillifl, 1928.; 370-0. G. 757).

The invention herein described-maybe-manufactured and used by or forthe Governmentfor,

lates to improvements in methods andapparatus for the crystallization of ammonium nitrate.

This application is a continuation-in-part of my copending application serialNo. 65 L490 filed March 14, 1946, now abandoned.

It has been customary in the art to accomplish continuous crystallization by suspending abody-of crystals in a rising current of liquid generated by introducing a stream,- of supersaturated solution. into a crystallizing vessel near the bottom thereof. As the; supersaturated solution rises in contact with. the body of crystals solute is deposited on the crystals and. the re,- sulting desupersaturated solution is withdrawn from an upper part of thevessel. The: introduction of a stream of supersaturated solution near the bottom of a crystallizing vessel has been employed to impart a swirling motion, to the suspension so that the crystals. pass repeatedly through the solution as it. is desupfirsaturated and growth is thus distributed evenly among the crystals in suspension. It is. customary to. control the rate of upward flow of solution through the crystallizing vessel to allow thelarger cI-ys.-. tals to settle to the bottom from whence they, are removed, and to prevent the smaller crystals from being carried out. of the top of the vessel by the overflow.

The known methods for continuous crystal lization ma be divided into two broad classes, namely, those in which new seed crystals are supplied from an outside source and those in which fresh nuclei are generated and grown along with the maturing crystals. United States patents covering processes of the first class are those of Isaachsen, Nos. 1,478,337 and 1,693,786 and of Howard, No. 1,559,703. Jeremiassens U. S. Patents Nos. 1,704,611; 1,860,741; 2,042,651; 2,164,111; 2,164,112; and 2,375,922 are directed to the second class although several of these patents describe both types of operation. However, the primary object of most known methods is to grow coarse crystals of desired size from seed crystals and little attention has been directed to the generation and control of growth of seed crystals.

In continuous processes crystals in suspension remain mobile at all times and numerous small fragments may be broken on by attrition and collision to serve as nuclei for new crystals. Other fresh nuclei may be produced in supersaturated solution because of the inocculating effect of existing crystals. The number of nuclei from both of these sources. can be regulated to some degree by varying the degree of agitation, the densit of crystal suspension in the crystallizing vessel, and the, degree of supersaturation. If excess nuclei are produced they may be separatedfrom the suspension and are usually dissolved, and recycled. If a. sufiicient. number of new nuclei is not generated seed crystals must be supplied from an outside source.

In order; to maintain efiicient, continuous operation o f a crystallizer producing; coarse granules, it is essential that only as many nuclei be retained as are required to equal the number of mature crystals withdrawn since each of theretained nuclei must be developed into a fully grown crystal.

The necessary balance of growth between large and small crystals has been accomplished in the prior art in the following three ways:

1. The small, crystals remain incorporated in a suspension of larger crystals at all times and thereb share with them the conditions favoring crystal growth.

2. The rate of desupersaturation of the supersaturated solution entering the suspension is sufiiciently slow that crystal growth is distributed widely throughout the suspension and therefore affects both large and small crystals even if the large and small crystals are segregated at various levels in the crystallizer.

3. The smaller crystals are caused to circulate through the supersaturated liquor circuit and therefore have an advantage in opportunity for growth over the larger crystals which are retained in the crystallization vessel at all times. These known methods for the generation and development of nuclei are adequate for the crystallization of substances characterized by slow rate of crystal growth, as such crystals can be produced in desired size and quality without very close control of the various factors involved. However, when it is desired to produce strong, smooth, coarse crystals of a substance characterized by rapid crystal growth, such as ammonium nitrate, sodium thiosulfate, or calcium nitrate, in large crystallizing vessels and at hi h produc n r t m thods o t prior art are v ry unsatisfac or The mere use of large crystallizing vessels introduces problems into the crystallization of su h sub tances. B c us of t e ra d stal rowth. only th e crys al in the imm d te vicinity of the point of introduction of supersaturated solution grow. All available solute is deposit d upon cr s als n h s lo t n-- In addition there is a ten ency f cr tal n suspension in a large vessel to accumulate in regions of greater crystal concentration thus leavins regions of lesse ys a conce ration o wells" extending upward through the solution. Since the incoming fluid has a tendency to follow the line of least resistance, it tends to flow U. S. Patent No. 1,573,716 has proposed the use of various difliusion elements in the path of the incoming solution of such nature that the resistance to flow of the solution through the'element is sufliciently in excess of the resistance to flow through the suspension so that the effect of minor variation in crystal density is minimized. In conjunction with these diffusion elements deflectors may also be used to impart horizontal velocity components to the solution which will sweep any static bed of crystals that tends to settle out into the incipient wells in the suspension, thereby filling them with crystals and counteracting the initial condition which tended to form the wells.

In addition, Jeremiassen in U. S. Patent No. 2,164,111 has proposed the use of specific formation of aggregate or crystals in the place of fixed elements to serve as diffusion elements required to obtain uniform distribution of solution.

While the present invention is applicable to crystallization generally, and is especially advantageous in the crystallization of all substances both organic and inorganic which are characterized by rapid crystal growth, it will be described with reference to the crystallization of ammonium nitrate, a material which has been crystallized with difficulty in the past.

When crystallization of ammonium nitrate is attempted by circulating nuclei through an extetroduction of the resulting larger crystals into the crystallization vessel proper as seed crystals, the crystals produced are weak, rough, and

- rior supersaturated liquor circuit and the ininferior to those developed and grown in the crystallizer only. As is explained in detail later, the degree of supersaturation necessary to obtain maximum production of finished crystals is such as to cause the small seed crystals to grow too fast while in the exterior circuit and as a result weak, intermediate-size crystals are produced. It is, therefore, apparent that the known methods are not adequate to produce strong,

" coarse crystals of ammonium nitrate at an economical production rate. For most commercial purposes it is necessary that crystals of ammonium nitrate be strong, coarse and that the production rate be high enough to make the process economical. By the methods of the prior art processes it has been possible to obtain these characteristics when working with crystals such as ammonium sulfate or other substance characterized by slow crystal growth. With ammonium nitrate or other material characterized by very rapid crystal growth, it has been necessary to sacrifice one or more of the desired characteristics in order to obtain the others. Strength of crystals and high production rate can be obtained but the crystals produced are too fine for fertilizer use or for many other commercial uses. Likewise, large crystals can be made at high production rate but they will be rough and weak; and strong, large crystals can be produced only at a low production rate.

It is an object of this invention to provide a method' for the rapid production of strong,

smooth, coarse crystals of a material characterized by rapid crystal growth.

Another object is to provide a method for 4 positive regulating of the growth of crystals at all stages from nuclei to maturity.

Another object is to provide a method for preventing irregular distribution of crystals in a growing zone.

Another object is to provide a method for producing ammonium nitrate crystals suitable for fertilizer use at high production rate.

Still anotherobject is to provide an appara tus for the production of crystals adapted to positively control the growth of crystals at all stages.

Another object is to provide an apparatus for producing ammonium nitrate crystals suitable for fertilizer use at high production rate.

Other objects and advantages will become apparent as this disclosure proceeds.

I have now found that these objects maybe easilyattained in apparatus embodying principles of my invention by continuously introducing a stream of supersaturated solution of the material to be crystallized into a lower part oi. a crystallizing vessel; directing said stream at predetermined or controlled velocity to maintain two zones in the resulting body of liquid in the crystallizing vessel, namely, a lower crystallizing zone wherein the liquid is maintained:- in substantially streamlined, substantially verti cal rotation and an upper relatively quiescent zone; introducing seed crystals into said lower crystallizing zone; producing fine crystals in said? crystallizing zone by attrition of coarser crystals; withdrawing mature crystals from a lower part of said crystallizing zone; withdrawing said fine crystals produced by attrition into said up per, relatively quiescent zone; contacting said fine crystals with supersaturated solution of the material being crystallized until their size is substantially increased; controlling the degree of supersaturation of saturated solution and introducing the resulting crystals of increased size into said crystallizing zone as seed crystals. The apparatus of my invention comprises in combination a crystallizing vessel; means for continuously introducing a stream of supersaturated solution at predetermined or controlled velocity into a lower part of said vessel; means for directing the entering stream of supersaturated solution operable in conjunction with said means for introducing supersaturated solution to maintain substantially vertical rotation of liquid in substantially streamlined flow in the lower portion of the crystallizing vessel, to maintain a zone of relatively quiescent liquid in the crystallizing vessel above the liquid in vertical rotation and to provide for continuous transfer of liquid from the rotating liquid to' the relatively quiescent liquid; means for introducing supersaturated liquid at controlled rate into said upper quiescent zone and means for withdrawing mature crystals from a lower part of the crystallizing vessel.

The accompanying drawings show diagrammatically three modifications of apparatus embodying principles of my invention and in which my novel process may be carried out. Figure l is a vertical section through a crystallizing vessel and shows a vacuum evaporator and other accessory equipment which may be used therewith; Figure 2 is a vertical section through a crystallizing vessel of different design and Figure 3 is a vertical section through three cryssaid last men ioned super- I nee ing eans fll 'isfdisposed v "la-cent to the t'a'p'ered end; 4, 3' of" the jtar'e etr c 1eg'an m cemmmun with the curved of vessel I10 is adapted ;to' "induce stream] flow ofliquid' in' the lower partbfcrystallizliig vessel It. A weir l5 isdiSpo'sed in an upper part of vessel Ill and is adapted to regulate liquid level" in the -cryst'alli'zing vesseL 'A' line l6 is attached to and communica tesavith -that art fto receive overflow "liquid andto conductil 'e samejto'punip l'lffrom whence itirna'y to'* evaporator l2"v'ialine {8. Means for removing mature crystals, shown ag'air lift I9, is

idi'sposed'in vessel 10 and adapted ,tdre'rho'iie a slu'rry of mature'crystals from the lower of the'vssel and to discharge the slurry'through line 20. A rnovable use-refit: restart n openin'gs122 is'disp'osedtosurround a portion of baro- Yntric lea'l'l 1'11 an upperpart qf vessel is adapted to be moved to bring openings 22 into coincidence with correspondingopening 23 in barometric leg I I. Vessel I!) may be circular in cross section or rectangular if'of very large capacity. In :,,the

latter case the tapered end I3 of the? barometric legis drawn out in fishtail shape along thetransverse axis of the vessel; While deflecting means 14 is correspondingly lengthened. Figure 2 shows an equivalent form of apparatus; "In ;this"em=- bodiment'oi' my invention a barometric leg 3 .l is constructed without openings in" thenpper part of crystallizing vessel .30; {This 1185; 3.!ends in .a flared portion 32 adapted, in combination with the curved bottom of'vesselifl .to induce streamlinedflbw in liquid in the lower portion of vessel es. A plurality of curved pipes 133.3 are'disposed with their lower ends adjacent ,to'flared portion 53 and extend into an upper part qfvessel These pipe are adapted to conduct liquid from a location near the bottom of Vessel ifland adjacent to flared portion}; .to an upper 'partof vessel 30, and to prevent contact of'such liquid with liquid and crystals inmedial poi"- tions of vessel 31!. Means 3! for removin mature crystals is disposed in vessel 3,0. Joint 135 is disposed in leg BI'adapted to be replaced with other joints of similar construction-to regulate l th Qf.

Figure 3 illustrates a multiple stage apparatus embodying "principles -ofmy'inventiorL- A 'plurality-ci crystallizing vessels A], 4.2; '43 are 'ar' ranged in a series characterized by diminishing size. Each crystallizing vesselis -connected"t'o and communicates with an adjacent vessel of the serie via-a line, as 75",44. Means 46, 47 for transferring crystals from a smaller to the next larger crystallizingvessel of thesei 'ies are singly disposed in'linesl 5; 44') A barometric leg 5"P'is disposed substantially" verticallyfin the" largest vessel M ofthe'serie'swith its' lowerend ad'iadfit to line 15. A line 48 is attached td'hfi'dbdfli' municates with barometric leg 51 and with lines 49, 5 0 singly disposed substantially vertically in each smaller vessel of the series near one side of thesrnallest'vessel 43'--ans ad acent toli l in intermediate vesel -Deflecting 52, '53 are disposed adjacent this; of barometric leg B'I an'd lines QS' -an was ass 51! .55 if? is dis ee is s'up ori;

was

aud jiie ry ale 1' ssnssssshc i 53s elected, 9 glef 111 th ower endi leg d .,A. ea 8. slurov le iortra s sr neliqm a u p r.par'tij f"'th ldis s 'r ssel 4. r thj rie tou per art sposed, adja ent t al rystal is "ad .ntgdwp r nd 31 slots to, .61. tiltli "t" ductia spanner/l u d .eo'vime through line T0: to a crystaltrap 13 com entcating with the interior of vessel ll atajsubs ial ycentr al at ninthe'bottomhtheifeof. I ttac e, jtof lid'jco H i "'ith a T3 rid is adapftdjto'conductfaslurry cr stal ,fioamtenp n if jee l e separate my. a m"'1 ,quigfl. not

Qsmti .I prefer to .begin operation, when apparatus comprising a single crys,talli zing vessel such as that shown in Fig.1 is used,:by introducing supersaturated solution through ,barometr'icleg 'l I and introducing sufficient seed crystals, in any manner; .to start crystallization."

' ,"Ihesupersaturated solution is .introduced at a controlled or predetermined velocity and rate of flow. "Such solution enterslfrom tapered (and 1:3 as a jetr'i'I'hisijetlis deflected by means l4 and .the walls ofvessel 1.0 to' cause the liquid and crystals suspended therein to" form two zones in vessel L0, namely, alo'wer zone'wherein .the liquid and'crystals are maintained in substantially vertical streamlined rotation at a velocity of about .31t0 10 tlmespthat' at which such crystals settle in such liquid, and an upper relatively quiescent zone. As crystals rotate with the solution in the lower zonefi'ne particles areibroken off by attrition. :The shape of the walls of vessel It! and defle'ctingme'ans' L4 are such as to'maintain substantiallystreamlineu flow in that part ofthe suspensionwhich .is'rotatin'g. It is, of course, impraticable to exclude all turbulence'from such rotationxbut no excessive'turbulence should be permitted as 'this would 'cause the formation of excessive numbers of" fine" crystal particles producedby attrition.

As theintroduction of supersaturated solution continues there 'is" continual transfer of liquid from "the lower zone of rotation to the upper, relatiyely quiescent zone. This transfer of liquid carries the fine crystal particles produced by'at trition into the relatively quiescent upper zone. Here additional supersaturated solution'is added penin s Z and 2.3 in s e ve 2! n leg H. Estefiios si slee 2! s em l yed to control th se 9 s mmit-states of the u er qu e cen Z6 pf 11 l idi s s e l errata! P r ic e u oiled con ti s until their Size "fluence of gravity until they rotating liquid in the lower zone and quickly grown to mature crystals.

Excess liquid from the upper quiescent zone overflows weir l and is carriedvia line [6, pump, l1 and line [8 to evaporator I2'where it isconcentrated and reintroduced into vessel via le II. A slurry of mature crystals is continuously withdrawn from vessel l0 via a conventional air lift l9 and is discharged via line 20. I

A similar method of operation is used with the apparatus shown in Fig. 2. Here the flared end, '32 and curved bottom of vessel 30 deflect the en,-,

terlng liquid to cause substantially vertical rotation of liquid in a lower zone overlaid with liquid in a relatively quiescent upper zone. The curved pipes 33 are adapted to receive a portion of the supersaturated liquid entering via leg 3| and to conduct the same to the upper quiescent zone or liquid. Control of the degree of supersaturation is, achieved by varying the number, size and posi tion of pipes 33. In this figure a lengthening joint 35 is shown inserted in legg3| and illustrates one convenient means for controlling the velocity 1 rotation of the major portion of the liquid at a velocity of about 3 to 10 times that with which "crystals settle in such liquid. After crystallization begins fine crystals produced by attrition move with upflow of liquid into an upper relatively quiescent zone in each vessel. From the upper zone of the largest vessel 4! liquid containing these fine crystals is transferred to the smallest vessel 43 and there the crystals grow in liquid of controlled degree of supersaturation. When these grow to sufficient size they sink into line 44 and are transferred by means 41, contra to fluid flow, into vessel 42. Here the crystals circulate in contact with liquid of a controlled degree of supersaturation and increase in size. When of a size determined by the velocity of rotation in'vessel 42 and the velocity of flow from vessel 4| to 42 via line the crystals sink into line 15 and are transferred as seed crystals to vessel 4! by means 46. The seed crystals then circulate in vessel'4l until mature. Mature crystals sink into salt trap 13 and are withdrawn as' a slurry via line 14.

From the upper quiescent zone in each'vessel liquid overflows weirs 63, 64 and 65 into trough 66 and is withdrawn therefrom via line 58, pump 69 and line 10 to an evaporator (not shown) or any other equipment desired. A portion of the liquid flowing through line 10 is withdrawn via line H containing a regulating valve 12 and is in-. troduced into salt trap 13 thus causing an upward flow of liquid in trap 13 and furnishing a further means of controlling the size of mature crystals.

In order to maintain in a crystallizing vessel two zones of the characteristics described, it is necessary that the velocity of the incoming solution be controlled or predetermined so that the stream imparts kinetic energy to the contents of the vessel at a rate not less than that at which potential energy is lost when such introduction are caught'by the of liquid is suddenly stopped. The minimum energy input will vary with the size and shape of crystals produced, the difference in density of crystals and solution, the viscosity of the solution and other smaller effects. The minimum requirements may be approximated from experimental measurements of the density of suspension for a given rate of flow through the suspension as is shown by the following example.

Example I Using ammonium nitrate granules of 8- to 10- mesh screen size (Tyler standard) at room temperature (20-25 0.), it was found that 30 pounds of crystals per cubic foot could be suspended in a current of saturated solution flowing at the rate of 0.06 cubic feet per second per square foot cross sectional area. This weight of crystals per cubic foot of suspension became less as the rate of circulation increased or as the par- "ticle size decreased, but the value specified above serves as a typical example.

The apparent density of the crystals was pounds per cubic foot. The-void space between the crystals was therefore The density of crystals in'a static bed was found to be 60 pounds per cubic foot. By a calculation similar to that above the void space was therefore found to be 0.4 cubic foot. In going from the dynamic statein which 30 pounds per cubic foot of crystals are held in suspensionto the static'state, 0.3 cubic foot of solution must therefore diifuse from a given mass of crystals.

=0.7 cubic foot In a suspension, crystals under the influence of gravity tend to settle downward causing the solution to diffuse upward. After a sudden interruption of the flow of solution to a suspension, transition of crystals from the dynamic to the static condition will start at the bottom, causing solution to difiuse out of the lowermost masses of crystals. This diffusing solution keeps the crystals above temporarily in suspension, and the transfer from the dynamic to the static state is therefore restricted to the lower zone of the dynamic part of the suspension. Experimental observation indicates that conditions in the upper part of the suspension remain essentially constant down to the zone in which crystals transfer from the dynamic to the static state. In the given example crystals will therefore settle out at such a rate that the supply of solution resulting from difiusion will produce an upward flow of 0.06

cubic foot per second per square foot of. area.

Since 0.3 cubic foot diffuses from 1 cubic foot of suspension, the rate at which the suspension settles out per square foot area is :02 cubic foot per second the apparent weight per cubic foot of suspension times the volume of the suspension times the difference in the position of the center of gravity of the crystals in suspension and after they have There- I allowed'to-Isettle OutL AssumingQa s ste sion '10 feet deep; .thecenter of 'gravity'would be {feet .above the bottom in the dynamic stat and 2.5[feet' above thebottom withthecrystals set tled into. a static. .bed. giving a difference. of... 2.5

feet. {Ihe,potential energy per square foot. would therefore be. 2. 5 10 X5.4=135 foot-pounds.

If therate of collapse is 0.2 cubic foot persec-. ond,. thetotaltime. of collapse is 5.0 seconds and the rate of decrease in potential 'energyiwhich must be, balanced by'an equal rate of kinetic energy input, is

amuse of power incorporated "in 0.06. cub icf f oot' ofv solution per second '(5 pounds) re quires a difference in pressure of 5 =0.5 l foot head or an initial velocity of H H (6 2 0.54 g). The peripheral velocity of the rotating suspension will be considerably lessthan this depending on wall friction and other retarding influences. A velocity of aboutltov 3 feet per second was observed in an experimental demonstration unit, whereas th relative velocity between crystals and solution is of the order of 0.1 to 0.3 foot persecond. a n

I have found that a principal factor affecting. the strength and smoothness of the crystalspro- 2.7 foot-pounds persec ond duced is the per cent rate of growth. which is defined forthis disclosure and claims as per cent of rate of growth:

hourly production rate, 1b./hr.

total effective suspension, lb. X 100 I have found that ammonium nitrate crystals grown at a 10 to 12 per cent rate of growth were dense, strong and had a smooth surface, whereas those grown at a 15 per cent rate of growth were ofpoor quality. In the present process the rate of .growth may be controlled by varying either the amountof crystals in suspension or the production'rate.

With the apparatus and conditions shown and described herein an adequate supply of seed crys-- tals averaging about 0.1 cubic millimeter in size is produced byattrition. I have found that the useof. seed crystals produced by attrition is another major factor leadingto the formation of T dense,.strong smooth crystals. The production about 6 feet pe r second crystallize this materialfllat about room' temperature. For. best operation the temperatureof; crystallization and room'temperature shouldbe approximately the same in order to minimize uncontrollable heat. transfer between the room and apfiaratusbut operation has been carried onusing an insulated.. crystallization vessel ,which gave excellent results when the difference. between crystallizes temperature and that of the room was .Qperation according I the proposed invention can be briefiy summarized as follows. The major portion. .ofthesuspension is causedhto rotate, at

. an average, velocity of about 1 to 3 feet per second,

the. .velocit y being least, atthe center of rotation andjncreasing progressivelyas the distance from the center becomes greater. The shape ofthe container is so designedas to allow the freehotation of the ,suspension, .Sincejhe" maximum velocityat which crystals settle out under the force of, gravity is less than 0.3 feet per second, thislrotational yelocity is sufliciently in excess of the settling rate to sweep crystals along with it.

The rotation is maintained at the proposed velocity by introducing a stream of solution tangentially into the rotating body at an approximate calculatedjet velocity of 6 to 10 feet per second. This jet is distributed uniformly over the bottom of the crystallizer container in one or more steps' to sweep along any crystals which .tendto settle from. the suspension. Since the solution and crystals rotate cocurrently, the relative velocity between the two remains small and permits the maintenance of a relatively dense suspension.

Efiicient utilization of the entire suspension is enhanced by a relatively high rate of rotation, while the maximum density .of suspension is obtained at low rates of rotation. In actual prac-. tice a compromise between :these two factors is involved,.giving a speed of rotation. at which a maximum production rate is obtained. This corresponds approximately to the conditions given above.

capacity of .apparatus for crystallizing ammo-;

density of the crystal suspension and the volume 'I of the effective zone of growth. .By operating according to themethod described above it is possible to. use a much higher degree of super.-.

saturation than has been feasible heretofore.

In the present type of operation ammonium nitrate solutions of from 0.003 to 0.10 pound per gallon supersaturation maybe used. Rotation of a zone of growing crystals distributes the su.-...,.

persatuigated liquid throughout-a large volume and dense suspensions of growing crystals may be used.

Thetemperature of crystallization is not critical 'I from. the. standpoint of operation; however,

since'arnmonium nitrate forms one type of crystal i liesmetween 0.4mm. 90.1 Ran at .tempe' W P Ammonium nitrate was crystallized without use of the rapidly rotating zone of crystal growth described above. A tilted plate similar to deflectingmeans 5| in Fig. 3 was then installed andcrystallization was continued with the; growing zone of crystalgrowth rotating substantially-ventcally in substantiallystreamlined flow at an avera'gevelocity of from 1 to 3 feet per second and overlaid by liquid in an upperquiescent zone- The following comparable results were obtained:

. Duration. o1':test 48 hr. Crystallization temperature, F 78-81. Rate ofrecirculation of solution, 90.

gals/min. I i Initial'supersaturation, lb./gal .007. Average rateo'f production of crys- 40-20.

tals,lb./hr. Weight of crystals insuspension, lb. 500-150. -Screen".ana1ys1s of product, per

cerium +8'mesh 45; .8 to l-12 meshLL; 39. l2.to +20 mesh 12. 20 to :35 m h 4. .,-;35'mesh 0.

Quality of product crys Dense, smooth,

and strong. I

eas; ate'foprase"waging? ceased; n; case. In eachcase the weight of crystals in .s lsaee 'iee.- eiin ll deqr d a .Qiz re ae. progressed, from 500 pounds to pounds and assists 150 pounds, respectively. The principal advantage gained was in the improved quality and size of crystals produced inthe rotating zone.

Example III 7 Operation was continued as described in Ex ample II with a rotating zone of crystal growth but with the addition of supersaturated solution at a controlled rate of about 10 gallons per minute to the upper quiescent zone. Fine crystal particles produced by attrition and present in this upper zone were thus grown at controlled rate until large enough to sink under the influence of gravity until caught by the rotating suspension of crystals in the lower zone. Steady state operation and high production rate were obtained without sacrifice of crystal size and quality.

7 Introduction of supersaturated solution into the upper quiescent zone was discontinued. Operationv with a rotating zone was continued as a check on the effect of introducing supersaturated solution into the upper quiescent zone. The following comparable results were obtained:

With in- Without intrcduction oi troduction of supersatusupersaturated solution rated solution Duration of test, hr 24 8 Temperature of crystallization, F 97 97 Weight of crystals in suspension, lb. 400-550 500-220 Recirculation of solution, ga1./min 105 100 Initial supersaturation, lb./gal 0. 008 0. 009 Average production rate, lb./hr 52 52 Screen analysis of product, per cent -]8 mesh l 2 8 to +12 mesh. 64 8 -12 to +20 mes H- 26 52 20 to +35 mesh 0 20 -35 mesh 0 18 When the introduction of supersaturated solution into the upper quiescent zone was discontinued the product contained an excessive proportion of fine crystals and the weight of crystals in suspension decreased so rapidly as to indicate that a high production rate could not be maintained. g

It will be obvious to those skilled in the art of crystallization that many modifications may be made in the apparatus and process shown and described without departing from the spirit and scope of this invention, which is limited only by the subtended claims.

Having described my invention and explained its operation, I claim:

1. A process for crystallizing a material characterized by rapid rate of crystal growth which comprises withdrawing solution of such material from the top of such solution contained in a crystallizing vessel; circulating the withdrawn solution through an evaporator; therein concentrating the solution to a selected degree of supersaturation; continuously introducing a stream of the resulting supersaturated solution into a lower part of said crystallizing vessel containing a suspension of crystals of said material in supersaturated solution thereof; directing said stream and controlling its velocity to maintain two zones in the resulting rising suspension of crystals, namely, a lower zone of crystal growth wherein a suspension of relatively coarser growing crystals is maintained in con tinuous rapid, substantially vertical streamlined rotation by kinetic energy imparted by said stream, and an upper, relatively quiescent zone containing a suspension of finer crystals; transferring fine crystals from said lower zone to said upper zone by upward flow of. solution: continuously introducing a controlled stream of supersaturated solutionof said material into said upper zone; therein contacting said fine crystals with resulting supersaturated solution of said material of a controlled degree of supersaturation until their size is substantially increased; introducing the resulting crystals of increased size into said lower zone as seed crystals; and withdrawing strong mature crystals from a lower portion of said lower zone. w

2. A process for crystallizing a material characterized by rapid rate of crystal growth which comprises withdrawing solution of such material from the top of such solution contained in a crystallizing vessel; circulating the withdrawn solution through an evaporator; therein concentrating the solution to a selected degree of supersaturation; continuously introducing a stream of the resulting supersaturated solution into a lower part of said crystallizing vessel containing a suspension of crystals of said material in supersaturated solution thereof; directing said stream and controlling its velocity to maintain two zones in the resulting rising suspension of crystals, namely, a lower zone of crystal growth wherein a suspension of relatively coarser growing crystals is maintained in continuous rapid, substantially vertical streamlined rotation by kinetic energy imparted by said stream, and an upper, relatively quiescent zone containing'a suspension of finer crystals; transferring fine crystals from said lower zone to said upper zone by upward flow of solution; withdrawing said fine crystals and introducing them into a secondary crystallization zone; therein contacting said fine crystals with supersaturated solution of said material of a controlled degree of supersaturation until their size is substantially increased; introducing the resulting crystals of increased size into said lower zone as seed crystals; and withdrawing strong mature crystals from a lower portion of said lower zone.

3. A process for crystallizing a material characterized by rapid rate of crystal growth which comprises withdrawing solution of such material from the top of such solution contained in a crystallizing vessel; circulating thewithdrawn solution through an evaporator; therein concentrating the solution to a selected degree of supersaturation; continuously introducing a stream of the resulting supersaturated solution into a lower. part oflsaid crystallizing vessel containing asuspension of crystals of said material in supersaturated solution thereof; directing said stream and controlling its velocity to maintain two zones in the resulting rising suspension of crystals, namely, a lower zone of crystal growth wherein a suspension of relatively coarser growing crystals is maintained in continuous rapid, substantially vertical streamlined rotation by kinetic energy imparted by said stream, and an upper, relatively quiescent zone containing a suspension of finer crystals; transferring fine crystals from said lower zone to said upper zone by upward flow of solution; withdrawing said fine crystals from said upper zone; introducing the withdrawn fine crystals into the first of a series of secondary crystallizing vessels; therein contacting. said fine crystals with supersaturated solution of said material of a controlled degree of supersaturation until their size is substantially increased; withdrawing the resulting crystals of increased size and introducing them in turn into each of the subsequent secondary crystallizing vessels in the series; contacting the resulting crystals of increasing size with supersaturated solution in each of the secondary crystallizing vessels in turn; withdrawing the resulting crystals of increased size from each of said vessels in turn; introducing the resulting crystals of increased size withdrawn from the last of said vessels in series into said lower zone as seed crystals; and withdrawing strong mature crystals from a lower portion of said lower zone.

4. A process for crystallizing ammonium nitrate which comprises withdrawing an aqueous solution thereof from the top of such solution contained in a crystallizing vessel; circulating the withdrawn solution through an evaporator; therein concentrating the solution to a selected degree of supersaturation; continuously introducing a stream of the resulting supersaturated solution into a lower part of said crystallizing vessel containing a suspension of ammonium nitrate crystals in supersaturated aqueous solution; directing said stream and controlling its velocity to maintain two zones in the resulting rising suspension of crystals, namely, a lower zone of crystal growth wherein a suspension containing relatively coarser growing crystals is maintained in substantially vertical streamlined rotation by kinetic energy imparted by said stream and at a speed of from about 3 to about times that with which such crystals sink in such solution, and an upper, relatively quiescent zone containing a suspension of finer crystals; transferring fine crystals from said lower zone to said upper zone by upward flow of solution; continuously introducing a controlled stream of supersaturated aqueous solution of ammonium nitrate into said upper zone; therein contacting said fine crystals with resulting supersaturated aqueous solution of ammonium nitrate of a controlled degree of supersaturation until their size is substantially increased; introducing the resulting crystals of increased size into said lower zone as seed crystals; and withdrawing strong, dense, mature crystals of ammonium nitrate from a lower portion of said lower zone.

5. A process for crystallizing ammonium nitrate which comprises withdrawing an aqueous solution thereof from the top of such solution contained in a crystallizing vessel; circulating the withdrawn solution through an evaporator; therein concentrating the solution to a selected degree of supersaturation; continuously introducing a stream of the resulting supersaturated solution, containing from 0.003 to 0.1 pound per gallon of ammonium nitrate in excess of the quantity required to saturate said solution, into a lower part of said crystallizing vessel containing a suspension of ammonium nitrate crystals in supersaturated aqueous solution; directing said stream and controlling its velocity to maintain two zones in the resulting rising suspension of crystals, namely, a lower zone of crystal growth wherein a suspension containing relatively coarser growingcrystals is maintained in ing a suspension of finer crystals; transferring fine crystals from said lower zone to said upper zone by upward fiow of solution; withdrawing said fine crystals from said upper zone; introducing the withdrawn fine crystals into the first of a series of secondary crystallizing vessels; therein contacting said fine crystals with supersaturated, aqueous solution of ammonium nitrate of a controlled degree of supersaturation until their size is substantially increased; withdrawing the resulting crystals of increased size and introducing them in turn into each of the subsequent secondary crystallizing vessels; contacting the resulting crystals of increasing size with supersaturated solution in each of the secondary crystallizing vessels in turn; withdrawing the resulting crystals of increased size from each of said vessels in turn; introducing the resulting crystals of increased size withdrawn from the last of said vessel in series into said lower zone as seed crystals; and withdrawing strong, dense, mature crystals of ammonium nitrate from a lower portion of said lower zone.

6. A process for crystallizing ammonium nitrate which comprises withdrawing an aqueous solution thereof from the top of such solution contained in a crystallizing vessel; circulating the withdrawn solution through an evaporator; therein concentrating the solution to a selected degree of, supersaturation; continuously introducing a stream of the resulting supersaturated solution into a lower part of said crystallizing vessel containing a suspension of ammonium nitrate crystals in supersaturated aqueous solu' tion; directing said stream and controlling its velocity to maintain two zones in the resulting rising suspension of crystals, namely, a lower zone of crystal growth wherein a suspension containing relatively coarser growing crystals is maintained in substantially vertical streamlined rotation by kinetic energy imparted by said stream and at a speed of from about 3 to about 10 times that with which such crystals sink in such solution, and an upper, relatively quiescent zone containing a suspension of finer crystals; transferring fine crystals from said lower zone to said upper zone by upward flow of solution; withdrawing said fine crystals and introducing them into a secondary crystallizing zone; therein contacting said fine crystals with supersaturated aqueous solution of ammonium nitrate of a controlled degree of supersaturation until their size is substantially increased; introducing the resulting crystals of increased size into said lower zone as seed crystals; and withdrawing strong,

' dense, mature crystals of ammonium nitrate from a lower portion of said lower zone.

substantially vertical streamlined rotation by kinetic energy imparted by said stream and at a speed of from about3 to about 10 times that with which such crystals sink in such solution, and an upper, relatively quiescent zone contain-- WALTER C. SAEMAN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,559,703 Howard Nov. 3, 1925 1,704,611 Jeremiassen Mar. 5, 1929 2,164,112 Jeremiassen June 27, 1939 2,424,207 Otto July 15, 1947 

1. A PROCESS FOR CRYSTALLIZING A MATERIAL CHARACTERIZED BY RAPID RATE OF CRYSTAL GROWTH WHICH COMPRISES WHITHDRAWING SOLUTION OF SUCH MATERIAL FROM THE TOP OF SUCH SOLUTION CONTAINED IN A CRYSTALLIZING VESSEL; CIRCULATING THE WITHDRAWN SOLUTION THROUGH AN EVAPORATOR; THEREIN CONCENTRATING THE SOLUTION TO A SELECTED DEGREE OF SUPERSATURATION; CONTINUOUSLY INTRODUCING A STREAM OF THE RESULTING SUPERSATURATED SOLUTION INTO A LOWER PART OF SAID CRYSTALLIZING VESSEL CONTAINING A SUSPENSION OF CRYSTALS OF SAID MATERIAL IN SUPERSATURATED SOLUTION THEREOF; DIRECTING SAID STREAM AND CONTROLLING ITS VELOCITY TO MAINTAIN TWO ZONES IN THE RESULTING RISING SUSPENSION OF CRYSTALS, NAMELY, A LOWER ZONE OF CRYSTAL GROWTH WHEREIN A SUSPENSION OF RELATIELY COARSER GROWING CRYSTALS IN MAINTAINED IN CONTINUOUS RAPID, SUBSTANTIALLY VERTICAL STREAMLINED ROTATION BY KINETIC ENERGY IMPARTED BY SAID STREAM, AND AN UPPER, RELATIVELY QUIESCENT ZONE CONTAINING A SUSPENSION OF FINER CRYSTAL; TRANSFERRING FINE CRYSTALS FROM SAID LOWER ZONE TO SAID UPPER ZONE BY UPWARD FLOW OF SOLUTION; CONTINUOUSLY INTRODUCING A CONTROLLED STREAM OF SUPERSATURATED SOLUTION OF SAID MATERIAL INTO SAID UPPER ZONE; THEREIN CONTACTING SAID FINE CRYSTALS WITH RESULTING SUPERSATURATED SOLUTION OF SAID MATERIAL OF A CONTROLLED DEGREE OF SUPERSATURATION UNTIL THEIR SIZE IS SUBSTANTIALLY INCREASED; INTRODUCING THE RESULTING CRYSTALS OF INCREASED SIZE INTO SAID LOWER ZONE AS SEED CRYSTALS; AND WITHDRAWING STRONG MATURE CRYSTALS FROM A LOWER PORTION OF SAID LOWER ZONE. 